The horseplay turned sour when Gronkjaer hurt an eye.
After seeing the team doctor the Chelsea star exchanged words with Bolton's Tofting before a scuffle ensued.
The pair wrestled each other to the ground, Tofting with his hand around his team-mate's throat.
An eyewitness said: "They were just horsing around but then suddenly it was a serious fight.
"It was over quickly, maybe five or six seconds, but it was a real fight - Tofting had his hand around his throat."
A Danish FA official said: "These players need a kindergarten teacher to sort them out."
It is the latest row to grab the headlines in the build-up to the finals in Japan and Korea.
Last week Freddie Ljungberg of Arsenal and Olof Mellberg of Aston Villa were involved in a clash on the training pitch.
Ireland's squad has also been rocked by the loss of captain Roy Keane after a dispute with manager Mick McCarthy.